We do not often think about the contents of the air that we breathe in on a daily basis. Perhaps it is time to start. Air pollutants too small to be seen, touched, or felt clearly affect our health. Beelen and colleagues have demonstrated that long-term exposure to fine particulate matter (pollution with particle diameter less than 2.5 μm) is associated with an increased incidence of natural-cause mortality. Disturbingly, the authors found the relationship to be present at concentrations significantly below regulatory standards, as defined by European annual mean limit values.

This is a well-designed study that used consistent statistical methods across multiple large subcohorts. The project leveraged data from the general population in 22 centers participating in the European Study of Cohorts for Air Pollution Effects (ESCAPE) project and assessed 5,118,039 person-years. The association between air pollution exposure and natural-cause mortality was examined separately in each group through assessments of pollution concentration, traffic intensity, total traffic on the nearest major road, and mortality registry data. Cohort-specific estimates were then combined by means of random effects meta-analysis. Multiple Cox proportional hazard models controlled for relevant confounders.

Particle pollution with diameter less than 2.5 μm was most consistently associated with mortality. Fine particulate matter has been previously implicated in the pathogenesis of oxidative stress and modulation of inflammation. These critical, phylogenetically preserved pathways may affect mortality and affect a host of disease processes. Cause-specific analysis data are not presented in this article and will be published separately, but the authors report increased hazard ratios for stroke mortality and lung cancer incidence. This is concerning, particularly for individuals who live in close proximity to high levels of traffic and vehicular exhaust. As studies confirming the detrimental health effects of air pollution continue to mount, changes in public policy may warrant consideration.

R. Beelen et al., Effects of long-term exposure to air pollution on natural-cause mortality: An analysis of 22 European cohorts within the multicentre ESCAPE project. Lancet, published online 6 December 2013 [10.1016/S0140-6736(13)62158-3]. [Full Text]